Over the past few decades, the development of lidars has great significance for ocean monitoring. The spaceborne single photon lidar shows its advantage in marine remote sensing because of high resolution and precision. In this paper, based on the response characteristics of single photon detectors and the sea surface reflection model, a theoretical model for quantitatively calculating the intensity of sea surface returns was proposed. Inputting the system parameters of the advanced topographic laser altimeter system (ATLAS) carried by ICESat-2 (the Ice, Cloud, and land Elevation Satellite 2), we verified the model with the echo intensity based on the measured point cloud data ATL03 from ICESat-2 and the global wind field data. The results show that this model performs well according to many tracks’ data above the Philippine Sea. When the sea surface wind speed is in the range of 4-10 m/s, the mean error is 0.0037 count/pulse, and the root mean square error is 0.153 count/pulse. The relationship between the echo intensity of the spaceborne single-photon ocean altimeter and the average wind speed above the sea level was analyzed. The conclusion provides an important theoretical basis for the wind speed inversion above the sea level.